Thermographic stencil

ABSTRACT

The invention provides a novel duplicating stencil comprising a backing sheet and attached to one edge thereof a thin sheet of thermoplastic film softening not above 250 degrees C. carrying a multiplicity of minute, closely spaced holes or artificially weakened areas. In use a heat absorbent image to be copied is placed in contact with the thermoplastic film and the assembly is exposed to heat, and the holes enlarge or the artificially weakened areas perforate in the image areas.

United States Patent Inventors Appl. No.

Filed Patented Assignee Priority Alexander Spencer Middlesex;

Alan Francis Blake; David Guthrie Burns, London, England 746,890

July 23, 1968 June 28, 1971 Gestetner Limited London, England July 25, 1967 Great Britain 34,075/67 THERMOGRAPHIC STENCIL 8 Claims, 7 Drawing Figs.

U.S.Cl l0l/128.3, 83/2, 96/364, 1 17135.5, 264/156 1nt.Cl 1141c 1/14 FieldotSearch 10l/128.3,

References Cited UNITED STATES PATENTS 1/1937 Hooper 2,550,366 4/1951 Meaker et a1 83/7X 3,012,918 12/1961 Schaar 264/156UX 3,092,439 6/1963 Harrison 264/156X 3,149,563 9/1964 Wartman et a1. 101/128.4X 3,267,847 8/1966 Hayama et al. l0l/128.4X FOREIGN PATENTS 618,181 2/1949 Great Britain 96/36.4 696,563 9/1953 Great Britain 101/1 28.4 707,160 4/1954 Great Britain 101/128.4

Pn'mary ExaminerRobert E. Pulfrey Assistant Examiner-C. Coughenour Attorney-Cushman, Darby and Cushman ABSTRACT: The invention provides a novel duplicating stencil comprising a backing sheet and attached to one edge thereof a thin sheet of thermoplastic film softening not above 250 C. carrying a multiplicity of minute, closely spaced holes or artificially weakened areas. in use a heat absorbent image to be copied is placed in contact with the thermoplastic film and the assembly is exposed to heat, and the holes enlarge or the artificially weakened areas perforate in the image areas.

1 THERMOGRAPHIC STENCIL This invention relates to duplicating stencils, which term is used herein to refer to unimaged duplicating stencils, stencils carrying images being specifically stated to be imaged.

It is known to make a heat-sensitive duplicating stencil from a sheet of orientated heat-shrinkable plastics film or from a sheet of heat-shrinkable plastics film laminated to a sheet of conventional stencil tissue (i.e. the highly porous paper made of long fibers which is conventionally used in the production of duplicating stencils designed to be imaged with a typewriter). lf a sheet carrying a heat-absorbent image is placed in contact with a duplicating stencil of either kind with the heat-sensitive film and the image in contact, and the assembly is then irradiatedso that the heat-absorbent image areas are heated relative to the remainder of the assembly, the heat-sensitive film shrinks and splits in the areas corresponding to the image, thus producing an imaged duplicating stencil which can be used. on a duplicator in the same way as a duplicating stencil prepared in the usual manner with a typewriter. The splits occur in the film because the shrinkage forces are greater than the tensile strength of the film. However, with duplicating stencils produced in this manner it is difficult adequately to regulate the degree to which the heatshrinkable film splits in the image areas. As a result, it is impossible accurately to control the sizes of the areas through which the ink can penetrate when the imaged duplicating stencil is used. One way which has been proposed for overcoming this difficulty is to use film which has a carefully controlled shrinkage, much lower than that of most commercially available heat-shrinkable films; but it is difficult to produce film having the necessary controlled degree of shrinkability. If the shrinkability is too great, the disadvantages already mentioned are not avoided, while if it is too small, the film does not perforate in use and an imaged stencil cannot be obtained.

It has now been found that a heat-sensitive stencil can be made without the use of specially controlled heat-shrink film by the use of orientated r nonorientated thermoplastic film, which carries a multiplicity of minute, closely spaced holes or artificially weakened areas. In use, if the film is heat-shrinkable, the said holes or areas ensure that the stencil perforates regularly in the image areas, while if the film is not heatshrinkable, it is surprisingly found that the said holes enlarge or the artificially weakened areas perforate in the image areas although under thesame conditions, but in the absence of the said holes or weakened areas, the film would not give a usable stencil.

carrying a multiplicity of minute closely spaced holes or artificially weakened areas, whereby the holes enlarge or the film Mark Cryovac S 254 (made from a 4:1 vinylidene chloride/vi- I nyl chloride copolymer produced by emulsion polymerizaperforates (as the case may be) in the image areas. These holes or perforations will then pass ink when the imaged stencil is used on a conventional stencil duplicator. In the case where the filminitially has holes, the enlarged holes in the image areas pass more ink, so' that the" desired image is produced, when the stencil is used, against a toned background, which. in many cases is unobjectionable or even desirable. When the film initially has weakened areas, the stencil perforates and only passes ink in the image areas, so that in use the image'is produced against an untoned plain background.

The thermoplastic film. may be any of the commercially available films which soften not above 250 C., whether orientated or nonorientated e.g. of polystyrene, vinylchloride/vinylidene chloride copolymers, high density polyethylene, polypropylene or polyethylene terephthalate. Suitable commercially available films include those sold under the Trade tion), .Cryovac S 205 or Saran film (made from a similar copolymer produced by suspension polymerization). Cryovac L (an irradiated polyethylene film), Cryovac Y (a biaxially orientated polypropylene film), Cryovac VPH (a monoaxially orientated polyvinyl chloride film), Sordelli-Vinifoil (a biaxially orientated polyvinyl chloridefilm), Melinex and Mylar films (both of polyethylene terephthalate, and polythene film (made of polyethylene). The thickness of the heat-shrinkable film is ordinarily of the order of 0.001 of an inch, say from one-fourth to twice this thickness.

Minute, artificially weakened areas in the film are conveniently (but not necessarily) made by embossing. The film is passed under pressure through thenip of a pair of rollers, one having a hard surface with minute projections, and the other having a plain, relatively resilient surface, e.g. of hard natural or synthetic rubber, cotton or compressed paper. This has the effect of reducing the thickness of the film around each projection thus creating weakened areas. The minute projections on the hard roller may be arranged regularly or irregularly. A suitable irregular pattern of projections is readily obtained by mounting a sheet of fine sand, emery or carborundum or other abrasive paper on a hard roller. Use of such an assembly gives a random patten of weakened areas on the embossed film, which is not necessarily disadvantageous. Alternatively a hard roller having a regular pattern of minute projections can be used. In this case, the number of projections as measured in each of two directions at right angles to each other, can be 60- --300 per inch, and is preferably from -200 per inch. Obviously the smaller and closer together the projections.(and hence the weakened areas) are, the better is the definition given by the resulting stencil, provided that the weakened areas do not tend to coalesce during use of the stencil. Practical consideration set an upper limit on the number of projections it is feasible to obtain. On the other hand, too few projections, and hence too large or too widely spaced weakened areas, give stencils which provide only poor quality copies.

The hard roller having the minute projections may be produced in conventional manner, e.g. by engraving a hard roller with a milled tool having the reverse pattern to that required on the roller. Alternatively the surface of the roller may be etched chemically into the desired pattern. Thesize of the projections naturally depends on their number. For example, at 200 projections per inch, their spacing will be 0.005 inch, and each projection should be about 0.004 inch high. The shape of the projections is not important, but will ordinarily be conical or pyramidal, with an apex angle below 45, e.g. 40.

As already mentioned, during use of the stencil, the weakened areas must not coalesce. This sets a lower limit on the spacing of the weakened areas, and hence on the spacing of the projections on the hard roller. Thus with 200 projections per inch, the diameter of each projection at the base will generally not be more than 0.004 inch, thus allowing 0.005 inch between projections, and hence between weakened areas in the embossed film. ln any particular case, the best spacing willdepend both on the physical properties of the material from which the film is made and also on precise size and shape of the projections on the roller. The best combination of spacing, height and diameter of the projections can be found in any particular case by routine experiment. v

For ease of production, itis sometimes preferable to ensure that the projections on the embossing roller completely penetrate the film; thus-giving'a sheet carrying'a multiplicity of minute holes, which preferablyvtaper so as to have a larger diameter on oneside of the film than the other. These holes may be left unsealed, in which case the stencil inuse gives (as already mentioned) the desired image against a toned background, or, more usually,-sealed by calendering or with a suitable plastics coating, or preferably both.

The material used to seal the perforations must be such as not to prevent reopening of the holes-in the image areas during imaging of the stencil. Thus it canbe polystyrene (used as an 0.5 to 20 percent solution in benzene), polyvinyl acetate and its copolymers (used as an 0.1 to 30 percent solution in industrial methylated spirit), vinyl chloride copolymers (used as an 0.5 to 10 percent solution in acetone), cellulose nitrate, acetate, or acetate/butyrate (each used as an 0.5 to 25 percent solution in acetone or ethyl acetate), polyvinyl butyral (used as an 0.l to 25 percent solution in 60:40 toluenezethanol), polyvinyl formal (used as an 0.15 to 30 percent solution in 60:40 toluenezethanol), and emulsions of similar polymers, e.g. polystyrene in water. The solvent in which the plastics material is applied must, of course, be selected so as not to attack the film, and the plastics material must adhere to the film before, during and afier use.

In many cases it is found that the embossed film is difficult to handle. It is therefore often convenient to laminate the film, before or after embossing, to a sheet of conventional stencil tissue containing to 30 percent of void area. The lamination may be effected in conventional manner for laminating sheets of synthetic thermoplastic resins to paper. When a material is used in the manner already described to seal perforations in the film, this material may also if desired act as an adhesive in the lamination.

The invention is further illustrated in the accompanying drawings, not to scale, in which FIG. 1 is a greatly enlarged, diagrammatic partial cross section showing a thermoplastic film about to be embossed or perforated;

FIG. 2 is a greatly enlarged, diagrammatic partial cross section showing a thermoplastic film after embossing;

FIG. 3 is a greatly enlarged diagrammatic partial cross section showing a thermoplastic film after perforation;

FIG. 4a is a greatly enlarged, diagrammatic partial cross section showing a thermoplastic film after perforation and subsequent calendering;

FIG. 4b is a greatly enlarged, diagrammatic partial plan view showing a thermoplastic film after perforation and subsequent calendering;

FIG. 5 is a greatly enlarged, diagrammatic partial cross section showing a thermoplastic film after perforation, calendering, application of a sealing coating and lamination to stencil tissue; and

FIG. 6 is a greatly enlarged, diagrammatic partial cross section showing the film of FIG. 5 after exposure to heat.

In FIG. 1, a single projection 2 from the surface of the hard roller is about to penetrate the surface of the thermoplastic film 1 to emboss or perforate the latter. FIG. 2 shows diagrammatically the embossed area 3 produced by a single projection 2. This area 3 is thinner and more highly stressed than the surrounding area of the film 1. It perforates when the film is heated.

FIG. 3 shows diagrammatically a single perforation 4 produced by a single projection 2 when impressed deeply enough to perforate rather than simply emboss the film l. The materialS displaced from the hole 4 remains attached to the film 1 on the reverse side from that through which the projection 2 enters. FIG. 4d shows the result of calendering the film of FIG. 3. The ni'atcrial 5 is forced back toward the hole 4 thereby at least partially closing it. FIG. 4b is a plan view of the calendered film. FIG. 5 shows the film of FIG. 4a after the application by conventional means of a sealing. coating 6 consisting of a thin layer of a suitable plastics material, and lamination to a sheet of stencil tissue 7. FIG. 6 shows the laminated film of FIG. 5 with the hole 4 reopened by the action of heat on the film l. y

The laminated or 'unlaminated sheet of thermoplastic film having minute holes or weakened areas may than be mounted in. the manner conventional for duplicating stencils by attaching one edge thereof to the edge of a backing sheet of similar size, the backing sheet being designed to be removed after the duplicating stencil has been imaged.

The duplicating stencil of this invention may be used in the following manner. The original'to be copied carrying a heatabsorbent image is placed in contact with the duplicating stencil so that the heat-absorbent image is in contact with the thermoplastic film. The assembly is then irradiated with infrared radiation which causes the image areas to become relatively hotter than the nonimage areas. This differential heating causes the weakened areas to rupture (or the minute holes to enlarge) and so produce ink-passing holes in the image areas. The imaged duplicating stencil is then used -in the same manner as duplicating stencils imaged in conventional ways.

The following Examples illustrate the invention.

EXAMPLE I Heat-shrinkable polystyrene film of thickness 0.0008 inch is passed through two rollers, one a backing roller with a cotton surface, the other a hard roller with 200 projections to the linear inch in each direction. The film is placed between the rollers and sufficient pressure is applied to enable the'projections of the hard rollers to penetrate nearly through the polystyrene film. This film is then sheeted to normal stencil size and mounted onto a suitable conventional backing paper.

EXAMPLE 2 Heat-shrinkable polyethylene 'terephthalate film (Trade Name Melinex (I.C.I.) or Mylar (Du Pont) 0.0005 inch thick or polystyrene film 0.001 inch thick is passed through the rollers used in Example I and sufficient pressure is applied to perforate the film. The perforated film is then passed through two plain rollers under pressure and the material displaced from the perforated holes is flattened so that the film is of relative even thickness throughout. The film is then lightly coated with a solution or aqueous dispersion such as one or other of the following.

Polyvinyl Acetate (60 percent solids) 10 parts by weight Industrial Ethanol 91.5 percent Water 8.5 percent (Industrial Methylated Spirit,

64 over proof) Polystyrene Aqueous Dispersion (Vinamul 710) 50 percent solids Water 10 parts by weight This solution or aqueous dispersion is coated onto the film and dried by a conventional technique. The coating weight is 0.5-3 grams per square meter.

The film is then sheeted'to normal stencil size and coated ontoa suitable conventional backing paper.

In Example 1, the hard roller maybe replaced by a roller covered with l600 grit size carborundum paper.

25 parts by weight 10 parts byweight EXAMPLE 3 Heat-shrinkable polyvinylidene chloride/vinyl chloride film, Trade name Cryovac 8.254 (W. R. Grace) 0.0004 inch thick is passed through embossing rollers as in Example 1 with sufficient pressure to perforate the film. This perforated film is laminated to conventional stencil tissue having a void area of 15 percent using a 10 percent solution of vinyl acetate in methylated spirit (e.g. Vinalac 5268, of Vinyl Products Limited). This laminated film is then sheeted to normal stencil size and mounted on to a suitable conventional backing paper.

EXAMPLE 4 Nonorientated polypropylene film of thickness 0.0005 inch is passed through the roller used in Example 1 under sufficient pressure to enable the projections of the hard roller to penetrate nearly through the polypropylene film. After embossing, the film is laminated to conventional stencil tissue having a void area of 15 percent or a similar matrix using one or other of the laminant adhesives specified in Example 2, which is coated onto the film and dried by a conventional technique. The coating weight is 0.S-7 grams per square meter.

This film is then sheeted to normal stencil size and mounted onto a suitable conventional backing paper.

The order of embossing and laminating is not critical, and, if desired, the lamination can take place before the embossing.

EXAMPLE 5 Example 4 is repeated except that during the embossing sufficient pressure is used to cause the projections of the hard roller to penetrate the polyropylene film. The perforated film is immediately coated with one or other of the adhesives mentioned in Example 2 at a coating weight of 0.5 to 7 grams per square meter. Before the coating is dry, the perforated film is laminated to a sheet of conventional stencil tissue.

After drying, the film is sheeted to normal stencil size and mounted onto a suitable conventional backing paper.

In Example 4, the hard roller may be replaced by a roller covered with l80-600 grit size carborundum paper.

We claim:

1. Method of producing an imaged duplicating stencil which comprises:

a. producing a multiplicity of minute, closely spaced artificially weakened areas in a sheet of thermoplastic film, said film having a softening temperature less than about 250 C.;

b. placing a heat-absorbent image to be copied in thermal contact with said film; and

c. exposing said image to heat and thereby producing ink transmitting perforations in the areas of said film which are in thermal contact with said heated image.

2. Method according to claim 1 wherein said artificially weakened areas are made by embossing.

3. Method according to claim 1 of producing 3,600 to 90,000 weakened areas per square inch of said film.

4. Method according to claim 3 for producing 22,500 to 40,000 weakened areas per square inch of said film.

5. Method according to claim 1, the additional step of selecting said thermoplastic film from the group consisting of polypropylene poly-(vinylidene/vinyl chloride), polystyrene and polyethylene terephthalate and thereafter producing said artificially weakened areas in said film.

6. Method according to claim I, the additional step of sealing said artificially weakened areas which consist of holes with a plastic coating.

7. Method according to claim 6 of selecting a plastic coating from the group consisting of polyvinyl acetate and polystyrene and thereafter sealing said artificially weakened areas with said coating.

8. Method according to claim 1, the additional step of laminating said film to a sheet of stencil tissue before placing said heat-absorbent image to be copied in thermal contact with said film. 

